1. Field of the Invention
The present invention relates to a multicast system, a communication apparatus, and a multicast method enabling a communication system that requires connection setup before multicasting to effectively utilize communications media, thus improving services.
2. Description of the Related Art
Internet protocols (IP) standardized by the Internet Engineering Task Force (IETF) have been widely used for recent communications. Examples of communications using IP include point-to-point communication and multicast communication. Multicast is a technology that enables the same data to be delivered simultaneously to a plurality of user terminals. In multicast, a user terminal informs an upstream network node of its desire to join or leave a certain multicast group. Upon receiving such a join or leave request for a multicast group from a user terminal, the upstream network node controls the delivery of multicast data to that user terminal.
The user terminal sends the join or leave request to the upstream network node by using a protocol, such as Internet Group Management Protocol (IGMP) or Multicast Listener Discovery (MLD). IGMP includes IGMPv1 and IGMPv2. In addition to the abilities of the IGMPv1, the IGMPv2 enables a leave request to be quickly sent to the upstream network node.
Besides, due to needs for broadbandization and availability of telecommunications infrastructure, various broadband technologies, such as wireless and passive optical networks (PON), have been increasingly used. In the PON's, light transmitted along an optical fiber is split by an optical coupler into a plurality of paths. The PON allows multiple user terminals to share the same optical fiber, while wireless technology allows multiple user terminals to share the same radio frequency. That is, with such technologies, communications media is shared by a plurality of user terminals, and therefore, communication data is relatively easily eavesdropped by a third party. For this reason, in the PON and the wireless technologies, a connection is established on media between nodes that communicate with each other, and data is logically sorted and managed according to the established connection.
In the PON and the wireless communication systems that require connection setup before multicasting, it is essential that a multicast control protocol, such as the IGMP and the MLD, which supports the network layer (Layer 3) of the OSI model be compatible with connection control in the data link layer (Layer 2) of the OSI model.
A conventional technique for multicast in the PON is disclosed in, for example, Japanese Patent Laid-Open No. 2004-343243. According to the technique, a station-side communication equipment: an optical line terminal (OLT), snoops an IGMP message (a join or leave request, etc.) sent from a user terminal, and performs signaling for connection control depending on the content of the message.
The conventional technique, however, has a disadvantage that media, such as an optical fiber and radio frequency, is consumed by signaling. A multicast-control message and a connection-control message are both control information that does not contain data to be provided to a user. In spite of this, the transmission of such control information through shared communications media causes the pressure on bandwidth for other users, thus reducing the quality of service.
Especially, in broadcast services, which are major services that use multicast, channel switching (zapping) involves the frequent transmission of both multicast-control messages and connection-control messages. The control messages result in the considerable waste of bandwidth. In addition, a plurality of control messages transmitted by one channel switching leads to a control delay.